In certain applications, it is important to be able to employ some form of dynamic power control over the power of a radio frequency (RF) signal transmitted by an RF transmitting system. One form of conventional transmitting system is shown in FIG. 1. With reference to FIG. 1, the system includes a transmitter, a radio frequency amplifier, a transmit antenna, and a controller. The transmitter converts base-band signals to a modulated intermediate radio frequency power and up-converts this power to a radio frequency which is suitable for transmission via licensed radio communications channels, and electronically-compatible with the selected transmit antenna. The radio frequency power input level to the transmit antenna is determined by the amplifier, as commanded by the controller. The transmission system may include additional feedback loops to the controller for more precise control of the transmit antenna's radio frequency output power.
The system above may be used to transmit information from an aircraft to a space-based RF receiving system, such as an RF transponder carried by a satellite. Alternatively, such an RF transmitter system may be used at a ground based location, for example, at an airport, to communicate with mobile platforms, such as aircraft parked at a gate or taxiing to or from a runway, or even with other mobile platforms such as motor vehicles operating at the airport. When such an RF system is employed on an aircraft, it is extremely important that the power level of the transmitted RF signal be monitored and maintained within predetermined power levels to avoid causing interference with other RF receiver systems disposed within the vicinity of the target RF receiver system which is the intended recipient of the RF signals being transmitted. With such a system, a controller must be employed to continually adjust the transmit power of the RF transmitter in order to avoid interference with non-target RF receiver systems. Such power control is also required to compensate for weather, aircraft/satellite geometry, and increases/decreases in information transfer rate. The hardware and software components of an RF transmitting system capable of providing the needed degree of precision RF power output control can therefore be very complex.
An additional concern is the need to limit RF exposure levels of persons located in the vicinity of the transmit antenna to assure transmission power levels are in compliance with RF radiation safety levels developed by the Federal Communications Commission (FCC) in the U.S., and by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) in Europe and elsewhere. When operating an RF transmitting system in ground-based applications, such as(for example) at an airport, the power flux density (PFD) of the radiated RF signal from the RF transmitter system must not under any circumstance exceed specific regulatory PFD levels. Although this safety-critical restriction can be technically maintained with the system shown in FIG. 1, formal certification of such a complex safety critical RF transmitter system can be impracticable due to the very high cost and considerable time required for such certification. Accordingly, there is significant need for an RF transmitter system that is capable of complying with strict regulatory safety requirements, but which still reduces and/or eliminates the need for the stringent safety critical level of certification for the majority of the system. Such an improved RF transmitting system would ideally require the stringent safety level of certification of fewer functionally partitioned subcomponent parts of the system and would therefore significantly reduce the time and cost associated with obtaining Federal Aviation Administration (FAA), FCC, ICNIRP, and other regulatory agency requirements for certification of the system.